Isotopes: Climate, Sea Level, Ecology

Transcription

1 Isotopes: Climate, Sea Level, Ecology

2 Definitions Isotopes Atoms of the same element (i.e., same number of protons and electrons) but different numbers of neutrons. Stable Isotope Do not undergo radioactive decay, but they may be radiogenic (i.e., produced by radioactive decay). Usually the number of protons and neutrons is similar, and the less abundant isotopes are often heavy, i.e., they have an extra neutron or two.

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4 Why are stable isotopes useful? Because of tiny differences in mass, different isotopes of a chemical element are be sorted by biological, chemical or physical processes. These naturally produced variations in isotope ratios are small (part per thousand), but easily measured. These differences in isotope ratio can be used as natural labels or tags. These differences can be used to monitor the rate or magnitude of processes.

5 What makes for a stable isotope system that shows large variation? 1) Low atomic mass 2) Relatively large mass differences between stable isotopes 3) Element tends to form highly covalent bonds 4) Element has more than one oxidation state or forms bonds with a variety of different elements 5) Rare isotopes aren t in too low abundance to be measured accurately

6 Since natural variations in isotope ratios are small, we use δ notation δ H X = ((R sample /R standard ) -1) x 1000 where R = heavy/light isotope ratio for element X and units are parts per thousand (or per mil, ) i.e., 10 = 1% + value = relatively more H than standard - value = relatively less H than standard δ 18 O is spoken aloud as delta O 18 Don t ever say del. Don t ever say parts per mil. These make you sound like a knuckle-head.

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8 Isotope Fractionation 1) Isotopes of an element have same number of protons and roughly the same number of electrons, hence they undergo the same chemical (and physical) reactions. 2) Differences in mass can, however, influence the rate or extent of chemical or physical reactions, or lead to partitioning of isotopes differentially among phases. 3) Isotopic sorting during chemical, physical, or biological processes is called Fractionation.

9 Fractionation mechanisms Equilibrium Isotope Fractionation A quantum-mechanical phenomenon, driven mainly by differences in the vibrational energies of molecules and crystals containing atoms of differing masses. Kinetic Isotope Fractionation Occur in unidirectional, incomplete, or branching reactions due to differences in reaction rate of molecules or atoms containing different masses.

10 Fractionation mechanisms Equilibrium Isotope Fractionation A quantum-mechanical phenomenon, driven mainly by differences in the vibrational energies of molecules and crystals containing atoms of differing masses. Kinetic Isotope Fractionation Occur in unidirectional, incomplete, or branching reactions due to differences in reaction rate of molecules or atoms containing different masses.

11 Fractionation terminology Fractionation factor: α A-B = H R A / H R B = ( δ H X A )/( δ H X B ) Discipline Term Symbol Formula Geochemistry Often equilibrium fractionations, put heavy isotope enriched substance in numerator Separation Δ A-B δ A - δ B Enrichment ε A-B 1000( A/B -1) Biology Often kinetic fractionations, put light isotope enriched substance in numerator Discrimination Δ A-B 1000( A/B -1) Enrichment ε A-B 1000 ln A/B Multiple Approximations 1000 lnα A-B δ A - δ B Δ A-B ε A-B

12 Climate and Isotopes Organisms sequester isotopes into their shells but fractionate them in constant or predictable manner CaCO 3 Seawater 13 C/ 12 C 18 O/ 16 O

13 Oxygen Isotopes & Water 16 O: % 17 O: % 18 O: %

14 δ 18 O/Temperature Calibration Experiment Temp d18oc-d18ow H 2 18 O + CaC 16 O 3 H 2 16 O + CaC 18 O 16 O lnα cc-water = (2.78x10 6 /T 2 )-2.89 T is in kelvin T C δ 18 Ocalcite-δ 18 Owater

15 Deep-sea Oxygen Isotope Record Benthic Foraminifera Minimal variations in temperature & salinity A record of global temperature and ice volume

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18 During H 2 O evaporation, 16 O concentrated in vapor Vapor pressure: H 2 16 O > H 2 18 O at 25 C, α l-v = if δ 18 O l = 0.0, then δ 18 O v = -9.2

19 18 O/ 16 O V / 18 O/ 16 O Vo = f α-1 where f is fraction of vapor remaining, and V o is initial vapor For vapor: δ 18 O V = (δ 18 O Vo )f (α-1) For rain: δ 18 O R = α(δ 18 O Vo ) -1000

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21 Bowen & Wilkinson (2002) Geology

22 Fricke & O Neil (1999) EPSL

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27 Sea-level Change 150 ka to present

28 120 meters

29 Carbon isotope correlation

30 How to measure the integrated isotope composition of vegetation? Bump et al. (2007) PRSB

31 Paleocene Eocene NALMA Age Age Age (Ma) (Ma) (Ma) Clarkforkian Wasatchian Paleocene Eocene Tiffanian Sub-Zones Polarity NALMA Wa-0 to Wa-3 Wa-4 Wa-6 Wa-7 Clarkforkian Wasatchian Paleocene Eocene Cf-1 Cf-2 Cf-3 Tiffanian Ti-5 Sub-Zones Polarity NALMA Wa-0 to Wa-3 Wa-4 Wa-6 Wa-7 25r 25n 24r 24n.3n Clarkforkian Wasatchian Cf-1 Cf-2 Cf-3 Tiffanian Ti-5 Sub-Zones Polarity Wa-0 to Wa-3 Wa-4 Wa-6 Wa-7 24n.2n 24n.2n 24n.2n 25r 25n 24r 24n.3n Wa-5 Wa-5 Wa-5 Cf-1 Cf-2 Cf-3 Ti r 25n 24r 24n.3n 52 Bighorn Bighorn Basin Basin soil soil soil carbonates Ma Clarks Clarks Fork Clarks Fork Basin Basin Fork Basin Central Central Bighorn Central Bighorn Basin Bighorn Basin Basin McCullough McCullough Peak Peak Peak δ 13 δc 13 Cδ 13 C 58 (PDB) (PDB) (PDB) C24 C23 C24 C25 C25 C26 C More 12 C Benthic Benthic foraminifera Benthic Benthic Marine Marine Foraminifera Foraminifera δ 13 Cδ 13 C δ 13 C (PDB) (, PDB) (PDB) More 13 C Early Eocene Warm Interval P-E Thermal P-E Thermal Maximum Colder 0.5 Koch et al. (2003) GSA Spec. Pub. 0 δ 18 O (, PDB) W -0.5

32 T. brandti T. belgica T. asiatica 20,000 km in ~20,000 years (living mammals 1 to 10 km/yr) Rapid speciation along dispersal Crossed Turgai and Bering Straits in all directions Smith et al. (2006) PNAS